Electrical and magnetic properties, room temperature optical absorption bands, and 300 and 90 K microcathodo luminescence (MCL) bands were studied in heavily Mn and Co doped (1-5 at. %) bulk ZnO crystals. Optical absorption bands near 1.9 eV (Co) and 2 eV (Mn) and MCL bands near 1.84 eV (Co) and 1.89 eV (Mn) are found to be associated with transition metal (TM) ions. These bands are assigned to internal transitions between the levels of the substitutional TM ions. The temperature dependence of the resistivity of the ZnO showed activation energies of similar to35 meV in all cases and the electron mobilities were decreased relative to the undoped material. (C) 2005 American Vacuum Society.